cv: initialize gerrit client in CL updater lazily.
cv: initialize gerrit client in CL updater lazily.

R=yiwzhang

Bug: 1170811
Change-Id: Ib11bdb07d1bb8beba1cfab1e3241ba3f536657cc
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2650525
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go